Securely Format a Mac Hard Drive

If you want to be absolutely sure your data is wiped clean with virtually no chance of recovery, by anyone, using any possible known recovery tools, look no further than Apple’s Disk Utility tool. The process is simple, and it can apply to any Mac drive, whether that’s an internal hard drive, external hard drive, and any connected drive of any format, meaning it does not need to just be a Mac drive to become securely formatted.

First a quick explanation of how secure format works: the drive is formatted and cleared of data as usual, but then the drive is rewritten with new randomly generated data, effectively overwriting any existing data on the drive and making it impossible to access or recover. It doesn’t stop there though, because that process is repeated multiple times, depending on which setting choice you select when securely formatting a drive. Let’s begin:

How to Secure Format a Mac Hard Disk Drive in OS X

Launch Disk Utility (located in /Applications/Utilities/)

Select the drive you want securely formatted

Click on the ‘Erase’ tab and click the “Security Options” button

You’ll see four available choices, the second two are what we’re looking to use

Select either 7-Pass Erase or 35-Pass Erase, depending on your needs

Click OK

Note about secure formatting boot volumes: if you’re looking to securely format the boot drive you will find it’s not accessible through Disk Utility by default. Instead, you will need to boot from another drive or from recovery mode and use secure erase from there.

7-Pass Erase is quite thorough and it meets the US Department of Defense standard for securely erasing media, by erasing the data then writing over it seven times. If the US DoD trusts it for privacy and security, we can expect that to be incredibly secure, but nonetheless there is an absolutely incredible level of security offered through the 35-Pass Erase option, which is even more intense and provides exceptional data deletion security by erasing the data then writing over it 35 times with random patterns of new data. That makes data recovery virtually impossible by any possibly known methods, and it’s theoretically 5 times more powerful than the 7 pass method.

Please note that because both 7 and 35 pass are writing over the data repeatedly, the length of time it takes to format the drive in this manner can be considerable (particularly 35 pass since it is doing this 35 times in a row), and it’s not unusual on a larger drive for this secure formatting process to take 24 hours. Thus, be prepared for a significant wait time with the stronger formatting options on larger hard drives. Drive speed will also effect how long it takes to secure format.

Of course, if you’re not looking to re-use the hard disk for another purpose, the absolute most secure known method of wiping a drives contents from the face of the earth is to first format the drive using an above 7-pass or 35-pass method, and then going a step further by simply physically destroying the drive itself in a thorough manner. Yes, actually destroying the hard drive, and people may use magnets, powerful incinerators, and perhaps most common, a plain old hammer and smashing the drive itself, to achieve the literal destruction of a disk drive and make it impossible to be used or recovered from. If you go the extreme route of drive destruction, pick a safe method and be sure to dispose of the resulting rubbish properly.

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5 Comments

The 35-pass erase option is generally considered pointless & a waste of time these days. Even at the time of writing in the ’90s the author of the 35-pass method said it was a ‘catch all’ to cover all types of HD technologies around at the time so was an overkill approach. Since then the author has criticised how his algorithm is over-prescribed for data scrubbing.
See:http://en.wikipedia.org/wiki/Gutmann_method#Criticism

One might also argue that the 7-pass is overkill too unless you truly believe that someone so intent on reading your data that they’ll crack open your hard disk and analyse the magnetic wave forms on the platters directly, for this is what it would take to read the previous data once the HD has truly overwritten it with some other data.

So unless you have paranoid delusions or are James Bond, the ‘zero out data’ option is just fine for the average user.

Of course, the last point in the OP about physically destroying the disk is still the most reliable method to erase data. Drills or Steamrollers spring to mind here. Though you might want to extract & save the magnets in the drive that control the heads, they’re incredibly strong for their size so can be really useful.

While doing this and selecting the Hard Drive, the option of SECURITY OPTIONS is grey and cannot be highlighted and under the FORMAT dropbox it reads MAC OS X (JOURNALED). Whats wrong or what is the next step to format the hard drive and perform a clean install?

[…] Options” button is greyed out in the Disk Utility options, seemingly preventing a standard “secure” erase procedure. The precise reason for this isn’t entirely clear, though some speculate it’s […]

[…] from an internal boot disk to an external drive of any type, you would be much better served using the secure format tools for an entire drive that are bundled within Disk Utility, which provide the option of 35-pass secure formatting. […]